This summer I've been dealing with 70 plus rescued garters of various ages and there are two foods that they have been willing to eat well. Earthworms is the #1 food they as well as my own pet garters, eat eagerly But recently, extremely dry weather conditions has made it tough to find areas in the woods that are damp enough for a productive earthworm dig so I turned to experimenting with a variety of other foods. The one food next to earthworms that the majority of the garters willingly eat, either 9 Lives meaty bites beef slices cat food or Friskies beef slices in gravy cat food-the two brands of this food are similar enough the snakes will eat either brand (one snake really astounded me with its show of intelligence, by reaching up when I was dropping a spoonful of the beef slices into a big paper plate for the snakes, and neatly taking a slice from the spoon. I offered a few more slices, one at a time, and this snake each time rose up and neatly took the slice from the spoon and repeated the behavior until it was satisfied. I tried to take a photo with the other hand, don't know if I managed to get a good pic or not coz I haven't had time to check the pics. If I didn't get a good pic, I'm sure going to try again because I doubt anyone, including myself until the day before yesterday, has ever seen a garter snake who taught itself to eat from a spoon!)
In addition five of these rescue garters decided I didn't have enough snakes to take care of and added over 150 little shoestrings to the population. A number of the baby garters willingly ate the beef cat food too although it has to be crumbled into really tiny pieces because a baby garter is easily intimidated by anything much wider than its own head.
I did have to cut the slices to make them thinner in width for the smaller snakes (yearlings and a few of the 2 year olds) to get them to eat and not be intimidated by the width of the food. Its strange that a garter will eagerly swallow a toad wider than itself but they are afraid to even try eating an earthworm that appears to be too large for them to swallow & from what I've seen so far, this seems to be true of everything I've seen a garter eat except for toads.
One note, try to wash the gravy off of the pieces of the food because the snakes eat the food more eagerly when they don't have to deal with the mess the gravy makes. This issue of gravy mess is even more important for babies, when there's more than just a tiny bit of gravy with the meat, they get sticky and have trouble moving around and have to be washed off. If they weren't washed off, they probably could become stuck to something because that gravy tends to have a gluelike effect when it dries.

Not only do most of the garters like it, but it was the one food that my handfed orphan baby robin was willing to accept in lieu of earthworms too, after the bird finally decided to eat on its own
Another food that I had some success with last year with a litter of baby garters was 9 Lives super supper bits rolled into shapes similar to earthworm shapes and once the babies figured out it was soft, they would willingly take bites out of larger chunks of it.

There WAS one other food the rescue garters liked when I tried it but after trying it, I decided it wasn't good for their health and that was canned smoked herring and because it was smoked and also quite salty, I discontinued it.

Foods I tried that the snakes rejected included canned tuna, tiny live fish in their swimming pool (I never have gotten a garter of mine to eat any kind of live fish, they will ignore the fish and let it live out its lifespan swimming among the snakes!), pinky parts, and chicken. Regarding canned fish, perhaps strong scent was why the garters would eat the herring.

I will post a separate thread detailing other things I discovered while working with these garters.
But you might try that beef slices cat food.